A conventional optical transmission system will be described below. As a conventional optical transmission system using FEC, a system achieved by a combination of an FEC multiplexing device and an FEC demultiplexing device described in ITU-T Recommendation G. 975 is known. FIG. 7(a) shows the configurations of the FEC multiplexing device, and FIG. 7(b) shows the configurations of the FEC demultiplexing device described in the above reference.
In FIG. 7(a), reference numeral 101 denotes a first demultiplexing circuit, reference numeral 102 denotes a second demultiplexing circuit, reference numeral 103 denotes a first rate charging circuit, reference numeral 104 denotes an OH insertion circuit, reference numeral 105 denotes an RS encoding circuit, reference numeral 106 denotes a first multiplexing circuit, and reference numeral 107 denotes a second multiplexing circuit. In FIG. 7(b), reference numeral 111 denotes a third demultiplexing circuit, reference numeral 112 denotes a fourth demultiplexing circuit, reference numeral 113 denotes a frame synchronizing circuit, reference numeral 114 denotes an RS decoding circuit, reference numeral 115 denotes an OH separation circuit, reference numeral 116 denotes a second rate change circuit 116, reference numeral 117 denotes a third multiplexing circuit, and reference numeral 118 denotes a fourth multiplexing circuit.
The operations of the FEC multiplexing device and the FEC demultiplexing device will be described below. The first demultiplexing circuit 101 which receives STM-16 data (2.5 Gbit/s) demultiplexes 16 parallel data (156 Mbit/s), and the second demultiplexing circuit 102 demultiplexes the received 16 parallel data into 128 parallel data (19 Mbit/s)
The first rate charging circuit 103 which receives the 128 parallel data adds redundant data regions to the data to generate 128 redundant parallel data (21 Mbit/s) The OH insertion circuit 104 inserts overhead information (e.g., frame synchronous information or the like) required to maintain/operate an optical transmission system into the 128 redundant parallel data. The RS (255, 239) encoding circuit 105 performs error correction encoding to an output from the OH insertion circuit 104.
The first multiplexing circuit 106 multiplexes the received data subjected to the error correction encoding into 16 parallel data (167 Mbit/s), and the second multiplexing circuit 107 generates an FEC frame (2.66 Gbit/s) from the 16 received parallel data.
On the other hand, the third demultiplexing circuit 111 of the FEC demultiplexing device which receives the FEC frame demultiplexes the frame into 16 parallel data (167 Mbit/s), and the fourth demultiplexing circuit 112 demultiplexes the 16 received parallel data into 128 parallel data (21 Mbit/s)
The frame synchronizing circuit 113 detects the start position of the FEC frame from the from synchronous information stored in the OH in the 128 received parallel data. The RS (255, 239) decoding circuit 114 detects an error of the data in the FEC frame and corrects the data into the original correct data.
The OH separation circuit 115 separates an OH from the corrected data, and the second rate change circuit 116 reduces the redundant regions to generate 128 parallel data (19 Mbit/s). The third multiplexing circuit 117 multiplexes the 128 received parallel data into 16 parallel data (156 Mbit/s), and the fourth multiplexing circuit 118 demodulates the original STM-16 data (2.5 Gbit/s) from the 16 received parallel data.
FIG. 8 includes diagrams showing the configurations of FEC frames generated by the FEC multiplexing device. The FEC frame is constituted by sub-frames 1 to 128 including one column of OH information, 238 columns of STM-16 data, and 16 columns of RS redundant data. For example, error correction encoding is performed every 8 sub-frames. More specifically, in the sub-frame 1 to 8, error correction code calculation is performed to the OH information and the STM-16 data, and RS (255, 239) redundant data are stored in R0-0 to R0-15 (see FIG. 8(a)). The FEC frame is generated by sequentially multiplexing the sub-frames 1 to 128 (see FIG. 8(b)). Reference symbol f (integer) in the FEC frame in FIG. 8(b) denotes the number of times of multiplexing of an RS code. FIG. 8(b) shows a case in which f=16 is satisfied.
In the FEC frame, since a transmission rate increases a rate which is 15/14 (255/238) the rate of the original STM-16 data, the transmission rate is 2.69 Gbit/s.
In this manner, in the conventional optical transmission system, the FEC frame is constituted as described above to make it possible to correct bit errors. As a result, high-quality service can be offered even if optical SNR decreases in the optical transmission system. In general, an RS (255, 239) code shown in FIG. 8 is changed into an RS (127, 111) code in which, for example, an error correction code length is reduced, i.e., the columns of the STM -16 data is changed from 238 to 110 (arbitrary integer equal to or smaller than 237) to increase the ratio of redundant information to information data, so that error correction capability can be more improved.
However, the conventional optical transmission system has the following problems. For example, when a distance for which transmission is to be performed is increased gradually, or when the number of wavelengths in a wavelength multiplexing system is increased gradually, so also the optical SNR deteriorates gradually. For this reason, the code length of an error correction code is reduced to maintain the error correction capability to some extent. On the other hand, when the code length of the error correction code is reduced, a ratio of redundant information to information data increases. For this reason, a transmission rate increases in accordance with the increase of the ratio. For example, when the rate of the STM-16 data is 2.5 Gbit/s, the transmission rate of an FEC frame subjected to RS (127, 111) encoding is 2.89 Gbit/s which is 127/110 times the rate of the STM-16 data.
For this reason, in the conventional optical transmission system, even though the code length of an error correction code is reduced to maintain error correction capability, an amount of deterioration of optical transmission characteristics is increased with an increase in rate, a long-distance/large-capacity optical transmission system having desired quality cannot be structured.
It is an object of the present invention to provide an optical transmission system which can improve error correction capability even though an amount of deterioration of optical transmission characteristics with an increase in rate. It is another object of this invention to provide an FEC multiplexing device constituting the optical transmission system, an FEC demultiplexing device, and a method of correcting error.